ISL6322_07 Datasheet, PDF (13/41 Pages) Intersil Corporation – Four-Phase Buck PWM Controller with Integrated MOSFET Drivers and I2C Interface for Intel VR10, VR11, and AMD Applications

ISL6322
Channel-Current Balance
One important benefit of multiphase operation is the thermal
advantage gained by distributing the dissipated heat over
multiple devices and greater area. By doing this the designer
avoids the complexity of driving parallel MOSFETs and the
expense of using expensive heat sinks and exotic magnetic
materials.
In order to realize the thermal advantage, it is important that
each channel in a multiphase converter be controlled to
carry equal amounts of current at any load level. To achieve
this, the currents through each channel must be sampled
every switching cycle. The sampled currents, I_n, from each
active channel are summed together and divided by the
number of active channels. The resulting cycle average
current, I_AVG, provides a measure of the total load-current
demand on the converter during each switching cycle.
Channel-current balance is achieved by comparing the
sampled current of each channel to the cycle average
current, and making the proper adjustment to each channel
pulse width based on the error. Intersil’s patented current-
balance method is illustrated in Figure 3, with error
correction for channel 1 represented. In the figure, the cycle
average current, I_AVG, is compared with the channel 1
sample, I₁, to create an error signal I_ER.
The filtered error signal modifies the pulse width
commanded by V_COMP to correct any unbalance and force
I_ER toward zero. The same method for error signal
correction is applied to each active channel.
V_COMP
+
-
FILTER f(s)
MODULATOR
RAMP
WAVEFORM
PWM1
+
-
I_ER
I_AVG
-
÷N
+
I₄
Σ
I₃
I₂
TO GATE
CONTROL
LOGIC
I₁
NOTE: CHANNEL 3 AND 4 ARE OPTIONAL
FIGURE 3. CHANNEL-1 PWM FUNCTION AND
CURRENT-BALANCE ADJUSTMENT
Continuous Current Sampling
In order to realize proper current-balance, the currents in
each channel are sensed continuously every switching
cycle. During this time the current-sense amplifier uses the
ISEN inputs to reproduce a signal proportional to the
inductor current, I_L. This sensed current, I_SEN, is simply a
scaled version of the inductor current.
PWM
SWITCHING PERIOD
I_L
I_SEN
TIME
FIGURE 4. CONTINUOUS CURRENT SAMPLING
The ISL6322 supports inductor DCR current sensing to
continuously sense each channel’s current for
channel-current balance. The internal circuitry, shown in
Figure 5 represents channel n of an N-channel converter.
This circuitry is repeated for each channel in the converter,
but may not be active depending on how many channels are
operating.
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
lumped quantity, as shown in Figure 5. The channel current
I_L, flowing through the inductor, passes through the DCR.
Equation 3 shows the s-domain equivalent voltage, V_L,
across the inductor.
V_L(s) = I_L ⋅ (s ⋅ L + DCR)
(EQ. 3)
A simple R-C network across the inductor (R₁ and C)
extracts the DCR voltage, as shown in Figure 5. The voltage
across the sense capacitor, V_C, can be shown to be
proportional to the channel current I_L, shown in Equation 4.
⎝⎛_D^--s---_C^{--⋅---LR--- + 1}⎠⎞
V_C(s) = ₍---_s----_â‹---_R-----₁----_â‹---_C------+-----₁----₎ ⋅ DCR ⋅ I_L
(EQ. 4)
In some cases it may be necessary to use a resistor divider
R-C network to sense the current through the inductor. This
can be accomplished by placing a second resistor, R₂,
across the sense capacitor. In these cases the voltage
across the sense capacitor, V_C, becomes proportional to the
channel current I_L, and the resistor divider ratio, K.
13
FN6328.1
February 15, 2007